YAP regulates cell mechanics by controlling focal adhesion assembly

Nuclear mechanosensing controls MSC osteogenic potential through HDAC epigenetic remodeling

Cells sense mechanical cues from the extracellular matrix to regulate cellular behavior and maintain tissue homeostasis. The nucleus has been implicated as a key mechanosensor and can directly influence chromatin organization, epigenetic modifications, and gene expression. Dysregulation of nuclear mechanosensing has been implicated in several diseases, including bone degeneration. Here, we exploit photostiffening hydrogels to manipulate nuclear mechanosensing in human mesenchymal stem cells (hMSCs) in vitro. Results show that hMSCs respond to matrix stiffening by increasing nuclear tension and causing an increase in histone acetylation via deactivation of histone deacetylases (HDACs). This ultimately induces osteogenic fate commitment. Disrupting nuclear mechanosensing by disconnecting the nucleus from the cytoskeleton up-regulates HDACs and prevents osteogenesis. Resetting HDAC activity back to healthy levels rescues the epigenetic and osteogenic response in hMSCs with pathological nuclear mechanosensing. Notably, bone from patients with osteoarthritis displays similar defective nuclear mechanosensing. Collectively, our results reveal that nuclear mechanosensing controls hMSC osteogenic potential mediated by HDAC epigenetic remodeling and that this cellular mechanism is likely relevant to bone-related diseases.

A Division of Labor between YAP and TAZ in Non-Small Cell Lung Cancer

Lung cancer is the leading cause of cancer-related deaths worldwide. The paralogous transcriptional cofactors Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ, also called WWTR1), the main downstream effectors of the Hippo signal transduction pathway, are emerging as pivotal determinants of malignancy in lung cancer. Traditionally, studies have tended to consider YAP and TAZ as functionally redundant transcriptional cofactors with similar biological impact. However, there is growing evidence that each of them also possesses distinct attributes. Here we sought to systematically characterize the division of labor between YAP and TAZ in non-small cell lung cancer (NSCLC), the most common histological subtype of lung cancer. Representative NSCLC cell lines as well as patient-derived data showed that the two paralogs orchestrated nonoverlapping transcriptional programs in this cancer type. YAP preferentially regulated gene sets associated with cell division and cell-cycle progression, whereas TAZ preferentially regulated genes associated with extracellular matrix organization. Depletion of YAP resulted in growth arrest, whereas its overexpression promoted cell proliferation. Likewise, depletion of TAZ compromised cell migration, whereas its overexpression enhanced migration. The differential effects of YAP and TAZ on key cellular processes were also associated with differential response to anticancer therapies. Uncovering the different activities and downstream effects of YAP and TAZ may thus facilitate better stratification of patients with lung cancer for anticancer therapies. SIGNIFICANCE: Thease findings show that oncogenic paralogs YAP and TAZ have distinct roles in NSCLC and are associated with differential response to anticancer drugs, knowledge that may assist lung cancer therapy decisions.

Interplay between Cell-Surface Receptors and Extracellular Matrix in Skin

Skin consists of the epidermis and dermis, which are connected by a specialized basement membrane-the epidermal basement membrane. Both the epidermal basement membrane and the underlying interstitial extracellular matrix (ECM) created by dermal fibroblasts contain distinct network-forming macromolecules. These matrices play various roles in order to maintain skin homeostasis and integrity. Within this complex interplay of cells and matrices, cell surface receptors play essential roles not only for inside-out and outside-in signaling, but also for establishing mechanical and biochemical properties of skin. Already minor modulations of this multifactorial cross-talk can lead to severe and systemic diseases. In this review, major epidermal and dermal cell surface receptors will be addressed with respect to their interactions with matrix components as well as their roles in fibrotic, inflammatory or tumorigenic skin diseases.

Functionalizable Antifouling Coatings as Tunable Platforms for the Stress-Driven Manipulation of Living Cell Machinery

Cells are continuously sensing their microenvironment and subsequently respond to different physicochemical cues by the activation or inhibition of different signaling pathways. To study a very complex cellular response, it is necessary to diminish background environmental influences and highlight the particular event. However, surface-driven nonspecific interactions of the abundant biomolecules from the environment influence the targeted cell response significantly. Yes-associated protein (YAP) translocation may serve as a marker of human hepatocellular carcinoma (Huh7) cell responses to the extracellular matrix and surface-mediated stresses. Here, we propose a platform of tunable functionable antifouling poly(carboxybetain) (pCB)-based brushes to achieve a molecularly clean background for studying arginine, glycine, and aspartic acid (RGD)-induced YAP-connected mechanotransduction. Using two different sets of RGD-functionalized zwitterionic antifouling coatings with varying compositions of the antifouling layer, a clear correlation of YAP distribution with RGD functionalization concentrations was observed. On the other hand, commonly used surface passivation by the oligo(ethylene glycol)-based self-assembled monolayer (SAM) shows no potential to induce dependency of the YAP distribution on RGD concentrations. The results indicate that the antifouling background is a crucial component of surface-based cellular response studies, and pCB-based zwitterionic antifouling brush architectures may serve as a potential next-generation easily functionable surface platform for the monitoring and quantification of cellular processes.

αV integrins in Schwann cells promote attachment to axons, but are dispensable in vivo

In the developing peripheral nervous system, Schwann cells (SCs) extend their processes to contact, sort, and myelinate axons. The mechanisms that contribute to the interaction between SCs and axons are just beginning to be elucidated. Using a SC-neuron coculture system, we demonstrate that Arg-Gly-Asp (RGD) peptides that inhibit α_V -containing integrins delay the extension of SCs elongating on axons. α_V integrins in SC localize to sites of contact with axons and are expressed early in development during radial sorting and myelination. Short interfering RNA-mediated knockdown of the α_V integrin subunit also delays SC extension along axons in vitro, suggesting that α_V -containing integrins participate in axo-glial interactions. However, mice lacking the α_V subunit in SCs, alone or in combination with the potentially compensating α₅ subunit, or the α_V partners β₃ or β₈ , myelinate normally during development and remyelinate normally after nerve crush, indicating that overlapping or compensatory mechanisms may hide the in vivo role of RGD-binding integrins.

BMP-2 Signaling and Mechanotransduction Synergize to Drive Osteogenic Differentiation via YAP/TAZ

Growth factors and mechanical cues synergistically affect cellular functions, triggering a variety of signaling pathways. The molecular levels of such cooperative interactions are not fully understood. Due to its role in osteogenesis, the growth factor bone morphogenetic protein 2 (BMP-2) is of tremendous interest for bone regenerative medicine, osteoporosis therapeutics, and beyond. Here, contribution of BMP-2 signaling and extracellular mechanical cues to the osteogenic commitment of C2C12 cells is investigated. It is revealed that these two distinct pathways are integrated at the transcriptional level to provide multifactorial control of cell differentiation. The activation of osteogenic genes requires the cooperation of BMP-2 pathway-associated Smad1/5/8 heteromeric complexes and mechanosensitive YAP/TAZ translocation. It is further demonstrated that the Smad complexes remain bound onto and active on target genes, even after BMP-2 removal, suggesting that they act as a "molecular memory unit." Thus, synergistic stimulation with BMP-2 and mechanical cues drives osteogenic differentiation in a programmable fashion.

Recombinant Human Soluble Thrombomodulin Suppresses Monocyte Adhesion by Reducing Lipopolysaccharide-Induced Endothelial Cellular Stiffening

Endothelial cellular stiffening has been observed not only in inflamed cultured endothelial cells but also in the endothelium of atherosclerotic regions, which is an underlying cause of monocyte adhesion and accumulation. Although recombinant soluble thrombomodulin (rsTM) has been reported to suppress the inflammatory response of endothelial cells, its role in regulating endothelial cellular stiffness remains unclear. The purpose of this study was to investigate the impact of anticoagulant rsTM on lipopolysaccharide (LPS)-induced endothelial cellular stiffening. We show that LPS increases endothelial cellular stiffness by using atomic force microscopy and that rsTM reduces LPS-induced cellular stiffening not only through the attenuation of actin fiber and focal adhesion formation but also via the improvement of gap junction functionality. Moreover, post-administration of rsTM, after LPS stimulation, attenuated LPS-induced cellular stiffening. We also found that endothelial cells regulate leukocyte adhesion in a substrate- and cellular stiffness-dependent manner. Our result show that LPS-induced cellular stiffening enhances monocytic THP-1 cell line adhesion, whereas rsTM suppresses THP-1 cell adhesion to inflamed endothelial cells by reducing cellular stiffness. Endothelial cells increase cellular stiffness in reaction to inflammation, thereby promoting monocyte adhesion. Treatment of rsTM reduced LPS-induced cellular stiffening and suppressed monocyte adhesion in a cellular stiffness-dependent manner.

Role of YAP/TAZ in Cell Lineage Fate Determination and Related Signaling Pathways

The penultimate effectors of the Hippo signaling pathways YAP and TAZ, are transcriptional co-activator proteins that play key roles in many diverse biological processes, ranging from cell proliferation, tumorigenesis, mechanosensing and cell lineage fate determination, to wound healing and regeneration. In this review, we discuss the regulatory mechanisms by which YAP/TAZ control stem/progenitor cell differentiation into the various major lineages that are of interest to tissue engineering and regenerative medicine applications. Of particular interest is the key role of YAP/TAZ in maintaining the delicate balance between quiescence, self-renewal, proliferation and differentiation of endogenous adult stem cells within various tissues/organs during early development, normal homeostasis and regeneration/healing. Finally, we will consider how increasing knowledge of YAP/TAZ signaling might influence the trajectory of future progress in regenerative medicine.

Disease-associated keratin mutations reduce traction forces and compromise adhesion and collective migration

Keratin intermediate filament (IF) proteins constitute the major cytoskeletal components in epithelial cells. Missense mutations in keratin 5 (K5; also known as KRT5) or keratin 14 (K14; also known as KRT14), highly expressed in the basal epidermis, cause the severe skin blistering disease epidermolysis bullosa simplex (EBS). EBS-associated mutations disrupt keratin networks and change keratinocyte mechanics; however, molecular mechanisms by which mutations shape EBS pathology remain incompletely understood. Here, we demonstrate that, in contrast to keratin-deficient keratinocytes, cells expressing K14R125C, a mutation that causes severe EBS, generate lower traction forces, accompanied by immature focal adhesions with an altered cellular distribution. Furthermore, mutant keratinocytes display reduced directionality during collective migration. Notably, RhoA activity is downregulated in human EBS keratinocytes, and Rho activation rescues stiffness-dependent cell-extracellular matrix (ECM) adhesion formation of EBS keratinocytes. Collectively, our results strongly suggest that intact keratin IF networks regulate mechanotransduction through a Rho signaling pathway upstream of cell-ECM adhesion formation and organized cell migration. Our findings provide insights into the underlying pathophysiology of EBS.This article has an associated First Person interview with the first author of the paper.

The liver fibrosis niche: Novel insights into the interplay between fibrosis-composing mesenchymal cells, immune cells, endothelial cells, and extracellular matrix

Liver fibrosis is a hepatic wound-healing response caused by chronic liver diseases that include viral hepatitis, alcoholic liver disease, non-alcoholic steatohepatitis, and cholestatic liver disease. Liver fibrosis eventually progresses to cirrhosis that is histologically characterized by an abnormal liver architecture that includes distortion of liver parenchyma, formation of regenerative nodules, and a massive accumulation of extracellular matrix (ECM). Despite intensive investigations into the underlying mechanisms of liver fibrosis, developments of anti-fibrotic therapies for liver fibrosis are still unsatisfactory. Recent novel experimental approaches, such as single-cell RNA sequencing and proteomics, have revealed the heterogeneity of ECM-producing cells (mesenchymal cells) and ECM-regulating cells (immune cells and endothelial cells). These approaches have accelerated the identification of fibrosis-specific subpopulations among these cell types. The ECM also consists of heterogenous components. Their production, degradation, deposition, and remodeling are dynamically regulated in liver fibrosis, further affecting the functions of cells responsible for fibrosis. These cellular and ECM elements cooperatively form a unique microenvironment: a fibrotic niche. Understanding the complex interplay between these elements could lead to a better understanding of underlying fibrosis mechanisms and to the development of effective therapies.

Extracellular Vesicles from Cancer-Associated Fibroblasts Containing Annexin A6 Induces FAK-YAP Activation by Stabilizing β1 Integrin, Enhancing Drug Resistance

Extracellular vesicles (EV) from cancer-associated fibroblasts (CAF) are composed of diverse payloads. Although CAFs impact the aggressive characteristics of gastric cancer cells, the contribution of CAF-EV to gastric cancer progression has not been elucidated. Here, we investigated the molecular mechanism of the changes in gastric cancer characteristics induced by CAF-EV. CAF abundance in gastric cancer tissues was associated with poor prognosis of patients with gastric cancer receiving chemotherapy. Moreover, CAF-EV induced tubular network formation and drug resistance of gastric cancer cells in the extracellular matrix (ECM). Comprehensive proteomic analysis of CAF-EV identified that Annexin A6 plays a pivotal role in network formation and drug resistance of gastric cancer cells in the ECM via activation of β1 integrin-focal adhesion kinase (FAK)-YAP. A peritoneal metastasis mouse model revealed that CAF-EV induced drug resistance in peritoneal tumors, and inhibition of FAK or YAP efficiently attenuated gastric cancer drug resistance in vitro and in vivo. These findings demonstrate that drug resistance is conferred by Annexin A6 in CAF-EV and provide a potential avenue for overcoming gastric cancer drug resistance through the inhibition of FAK-YAP signaling in combination with conventional chemotherapeutics. SIGNIFICANCE: This study elucidates a novel molecular mechanism through which Annexin A6 in CAF-EV activates FAK-YAP by stabilizing β1 integrin at the cell surface of gastric cancer cells and subsequently induces drug resistance. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/16/3222/F1.large.jpg.

Cellular architecture response to aspect ratio tunable nanoarrays

Nanoarrays have been emerging as popular nanostructure platforms to investigate both cell behaviors and biological functions, due to the cell architecture respondence to the biointerface of nanostructures. Herein, we developed a series of aspect ratio tunable nanoarrays through a metal-assisted chemical etching method. Nanoarrays including nanoneedles, nanopillars, and nanoclusters were fabricated with a controllable aspect ratio. We found that nanostructures with a high aspect ratio (>10) induced significant alterations of cell physiological behaviors such as surface attachment, architecture deformation, viability, proliferation and motility. The cells on nanostructures with a high aspect ratio exhibited reorganized actin stress fibers and vimentin filaments, as well as reduced focal adhesion. This research enlarges the diversity of nanostructures on nano-bio interface investigation, provides a new insight for the surface-dependent architecture of cells, and offers unbiased understanding of factors influencing cell physiology.

Quantitative Phosphoproteomics Reveals Cell Alignment and Mitochondrial Length Change under Cyclic Stretching in Lung Cells

Lung cancer is a leading cause of death. Most previous studies have been based on traditional cell-culturing methods. However, lung cells are periodically subjected to mechanical forces during breathing. Understanding the mechanisms underlying the cyclic stretching induced in lung cells may be important for lung cancer therapy. Here, we applied cyclic stretching to stimulate the continual contraction that is present under physiological conditions in lung cells. We first uncovered the stretching-induced phosphoproteome in lung cancer cell line A549 and fibroblast cell line IMR-90. We identified 2048 and 2604 phosphosites corresponding to 837 and 1008 phosphoproteins in A549 and IMR-90, respectively. Furthermore, we combined our phosphoproteomics and public gene expression data to identify the biological functions in response to cyclic stretching. Interestingly, cytoskeletal and mitochondrial reorganization were enriched. We further used cell imaging analysis to validate the profiling results and found that this physical force changed cell alignment and mitochondrial length. This study not only reveals the molecular mechanism of cyclic stretching but also provides evidence that cell stretching causes cellular rearrangement and mitochondrial length change.

Bad Neighborhood: Fibrotic Stroma as a New Player in Melanoma Resistance to Targeted Therapies

Current treatments for metastatic cutaneous melanoma include immunotherapies and drugs targeting key molecules of the mitogen-activated protein kinase (MAPK) pathway, which is often activated by BRAF driver mutations. Overall responses from patients with metastatic BRAF mutant melanoma are better with therapies combining BRAF and mitogen-activated protein kinase kinase (MEK) inhibitors. However, most patients that initially respond to therapies develop drug resistance within months. Acquired resistance to targeted therapies can be due to additional genetic alterations in melanoma cells and to non-genetic events frequently associated with transcriptional reprogramming and a dedifferentiated cell state. In this second scenario, it is possible to identify pro-fibrotic responses induced by targeted therapies that contribute to the alteration of the melanoma tumor microenvironment. A close interrelationship between chronic fibrosis and cancer has been established for several malignancies including breast and pancreatic cancers. In this context, the contribution of fibrosis to drug adaptation and therapy resistance in melanoma is rapidly emerging. In this review, we summarize recent evidence underlining the hallmarks of fibrotic diseases in drug-exposed and resistant melanoma, including increased remodeling of the extracellular matrix, enhanced actin cytoskeleton plasticity, high sensitivity to mechanical cues, and the establishment of an inflammatory microenvironment. We also discuss several potential therapeutic options for manipulating this fibrotic-like response to combat drug-resistant and invasive melanoma.

Combining Nanomaterials and Developmental Pathways to Design New Treatments for Cardiac Regeneration: The Pulsing Heart of Advanced Therapies

The research for heart therapies is challenged by the limited intrinsic regenerative capacity of the adult heart. Moreover, it has been hampered by the poor results obtained by tissue engineering and regenerative medicine attempts at generating functional beating constructs able to integrate with the host tissue. For this reason, organ transplantation remains the elective treatment for end-stage heart failure, while novel strategies aiming to promote cardiac regeneration or repair lag behind. The recent discovery that adult cardiomyocytes can be ectopically induced to enter the cell cycle and proliferate by a combination of microRNAs and cardioprotective drugs, like anti-oxidant, anti-inflammatory, anti-coagulants and anti-platelets agents, fueled the quest for new strategies suited to foster cardiac repair. While proposing a revolutionary approach for heart regeneration, these studies raised serious issues regarding the efficient controlled delivery of the therapeutic cargo, as well as its timely removal or metabolic inactivation from the site of action. Especially, there is need for innovative treatment because of evidence of severe side effects caused by pleiotropic drugs. Biocompatible nanoparticles possess unique physico-chemical properties that have been extensively exploited for overcoming the limitations of standard medical therapies. Researchers have put great efforts into the optimization of the nanoparticles synthesis and functionalization, to control their interactions with the biological milieu and use as a viable alternative to traditional approaches. Nanoparticles can be used for diagnosis and deliver therapies in a personalized and targeted fashion. Regarding the treatment of cardiovascular diseases, nanoparticles-based strategies have provided very promising outcomes, in preclinical studies, during the last years. Efficient encapsulation of a large variety of cargos, specific release at the desired site and improvement of cardiac function are some of the main achievements reached so far by nanoparticle-based treatments in animal models. This work offers an overview on the recent nanomedical applications for cardiac regeneration and highlights how the versatility of nanomaterials can be combined with the newest molecular biology discoveries to advance cardiac regeneration therapies.

Matrix mechanotransduction mediated by thrombospondin-1/integrin/YAP in the vascular remodeling

The extracellular matrix (ECM) initiates mechanical cues that activate intracellular signaling through matrix-cell interactions. In blood vessels, additional mechanical cues derived from the pulsatile blood flow and pressure play a pivotal role in homeostasis and disease development. Currently, the nature of the cues from the ECM and their interaction with the mechanical microenvironment in large blood vessels to maintain the integrity of the vessel wall are not fully understood. Here, we identified the matricellular protein thrombospondin-1 (Thbs1) as an extracellular mediator of matrix mechanotransduction that acts via integrin αvβ1 to establish focal adhesions and promotes nuclear shuttling of Yes-associated protein (YAP) in response to high strain of cyclic stretch. Thbs1-mediated YAP activation depends on the small GTPase Rap2 and Hippo pathway and is not influenced by alteration of actin fibers. Deletion of Thbs1 in mice inhibited Thbs1/integrin β1/YAP signaling, leading to maladaptive remodeling of the aorta in response to pressure overload and inhibition of neointima formation upon carotid artery ligation, exerting context-dependent effects on the vessel wall. We thus propose a mechanism of matrix mechanotransduction centered on Thbs1, connecting mechanical stimuli to YAP signaling during vascular remodeling in vivo.

Matrix stiffness controls cardiac fibroblast activation through regulating YAP via AT1 R

Cardiac fibrosis is a common pathway leading to heart failure and involves continued activation of cardiac fibroblasts (CFs) into myofibroblasts during myocardium damage, causing excessive deposition of the extracellular matrix (ECM) and thus increases matrix stiffness. Increasing evidence has shown that stiffened matrix plays an important role in promoting CF activation and cardiac fibrosis, and several signaling factors mediating CF mechanotransduction have been identified. However, the key molecules that perceive matrix stiffness to regulate CF activation remain to be further explored. Here, we detected significantly increased expression and nuclear localization of Yes-associated protein (YAP) in native fibrotic cardiac tissues. By using mechanically regulated in vitro cell culture models, we found that a stiff matrix-induced high expression and nuclear localization of YAP in CFs, accompanied by enhanced cell activation. We also demonstrated that YAP knockdown decreased fibrogenic response of CFs and that YAP overexpression promoted CF activation, indicating that YAP plays an important role in mediating matrix stiffness-induced CF activation. Further mechanistic studies revealed that the YAP pathway is an important signaling branch downstream of angiotensin II type 1 receptor in CF mechanotransduction. The findings help elucidate the mechanism of fibrotic mechanotransduction and may contribute to the development of new approaches for treating fibrotic diseases.

YAP/TAZ are Activated by Mechanical and Hormonal Stimuli in Myometrium and Exhibit Increased Baseline Activation in Uterine Fibroids

Uterine fibroids (UFs) are benign myometrial neoplasms. The mechanical environment activates signaling through the Hippo pathway effectors Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding domain (TAZ) in other fibrotic disorders. Here, we assess the differences in YAP/TAZ responsiveness to signals in UF compared with myometrium (Myo). Matched samples of UF and Myo were collected. Atomic force microscopy (AFM) was used to determine in situ stiffness. Cells were plated sparsely on hydrogels or at confluence. Ten nanomolars of estradiol (E2) and 100 nM progesterone (P4) were used. Immunostaining for YAP/TAZ and extracellular matrix (ECM) proteins was performed. Cells were incubated with control or YAP1 (YAP)/WWTR1 (TAZ) small interfering RNA (siRNA). Real time qPCR was completed for connective tissue growth factor (CTGF). Cells were treated with verteporfin (a YAP inhibitor) or Y27632 (a ROCK inhibitor), and ECM gene expression was analyzed. Paired t test and Wilcoxon sign-rank test were used. AFM-measured tissue stiffness and YAP/TAZ nuclear localization in situ and in confluent cells were higher in UF compared with Myo (p < 0.05). Decreasing substrate stiffness reduced YAP/TAZ nuclear localization for both Myo and UF (p = 0.05). Stimulating cells with E2 or P4 increased YAP/TAZ nuclear localization, but only in Myo (p = 0.01). UFs had increased FN, COLI, and COLIII deposition. Following siRNA targeting, CTGF was found to be statistically decreased. Verteporfin treatment reduced cell survival and reduced FN deposition. Treatment with Y27632 demonstrated better cell tolerance and a reduction in ECM deposition. The mechanosensitive pathway may be linked to YAP/TAZ function and involved in transducing fibroid growth.

Substrate Topography Regulates Differentiation of Annulus Fibrosus-Derived Stem Cells via CAV1-YAP-Mediated Mechanotransduction

Regeneration of annulus fibrosus (AF) through tissue engineering techniques shows promise as a treatment for patients with degenerative disc disease (DDD). Yet, it remains challenging because of the intrinsic heterogeneity of AF tissue and shortage of in-depth knowledge of its structure-function correlation. In the current study, we fabricated fibrous poly(ether carbonate urethane)urea (PECUU) scaffolds with various fiber sizes to mimic the microstructural feature of native AF and aimed to regulate the differentiation of AF-derived stem cells (AFSCs) by controlling the topographical cues of the scaffold. We found that the morphology of AFSCs varied significantly on scaffolds with various fiber sizes. Meanwhile, the expression of the phenotypic marker genes of outer AF was up-regulated on scaffolds with large fibers. Meanwhile, enhanced expression of the phenotypic marker genes of inner AF was seen on scaffolds with small fibers. Such topography-dependent gene expression in AFSCs approximated the biochemical profile of AF tissue in various zones. Moreover, cell spreading and nucleus translocation of Yes-associated protein (YAP) were facilitated with increased fiber size. Formation and maturation of focal adhesions of AFSCs were also promoted. We also found that Caveolin-1 (CAV1) positively modulated the mechano-responses of YAP in response to substrate topography. In conclusion, depending on the activation of the CAV1-YAP mechanotransduction axis, tuning the fiber size of scaffolds can effectively induce changes in cell shape, adhesions, and extracellular matrix expression. This work may therefore provide new insights in the design of novel materials toward AF tissue regeneration.

Hypergravity Activates a Pro-Angiogenic Homeostatic Response by Human Capillary Endothelial Cells

Capillary endothelial cells are responsible for homeostatic responses to organismic and environmental stimulations. When malfunctioning, they may cause disease. Exposure to microgravity is known to have negative effects on astronauts’ physiology, the endothelium being a particularly sensitive organ. Microgravity-related dysfunctions are striking similar to the consequences of sedentary life, bed rest, and ageing on Earth. Among different countermeasures implemented to minimize the effects of microgravity, a promising one is artificial gravity. We examined the effects of hypergravity on human microvascular endothelial cells of dermal capillary origin (HMEC-1) treated at 4 g for 15 min, and at 20 g for 15 min, 3 and 6 h. We evaluated cell morphology, gene expression and 2D motility and function. We found a profound rearrangement of the cytoskeleton network, dose-dependent increase of Focal Adhesion kinase (FAK) phosphorylation and Yes-associated protein 1 (YAP1) expression, suggesting cell stiffening and increased proneness to motility. Transcriptome analysis showed expression changes of genes associated with cardiovascular homeostasis, nitric oxide production, angiogenesis, and inflammation. Hypergravity-treated cells also showed significantly improved motility and function (2D migration and tube formation). These results, expanding our knowledge about the homeostatic response of capillary endothelial cells, show that adaptation to hypergravity has opposite effect compared to microgravity on the same cell type.

The G Protein-Coupled Estrogen Receptor (GPER) Expression Correlates with Pro-Metastatic Pathways in ER-Negative Breast Cancer: A Bioinformatics Analysis

The G protein-coupled estrogen receptor (GPER, formerly known as GPR30) is a seven-transmembrane receptor that mediates estrogen signals in both normal and malignant cells. In particular, GPER has been involved in the activation of diverse signaling pathways toward transcriptional and biological responses that characterize the progression of breast cancer (BC). In this context, a correlation between GPER expression and worse clinical-pathological features of BC has been suggested, although controversial data have also been reported. In order to better assess the biological significance of GPER in the aggressive estrogen receptor (ER)-negative BC, we performed a bioinformatics analysis using the information provided by The Invasive Breast Cancer Cohort of The Cancer Genome Atlas (TCGA) project and Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) datasets. Gene expression correlation and the statistical analysis were carried out with R studio base functions and the tidyverse package. Pathway enrichment analysis was evaluated with Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway on the Database for Annotation, Visualization and Integrated Discovery (DAVID) website, whereas gene set enrichment analysis (GSEA) was performed with the R package phenoTest. The survival analysis was determined with the R package survivALL. Analyzing the expression data of more than 2500 primary BC, we ascertained that GPER levels are associated with pro-migratory and metastatic genes belonging to cell adhesion molecules (CAMs), extracellular matrix (ECM)-receptor interaction, and focal adhesion (FA) signaling pathways. Thereafter, evaluating the disease-free interval (DFI) in ER-negative BC patients, we found that the subjects expressing high GPER levels exhibited a shorter DFI in respect to those exhibiting low GPER levels. Overall, our results may pave the way to further dissect the network triggered by GPER in the breast malignancies lacking ER toward a better assessment of its prognostic significance and the action elicited in mediating the aggressive features of the aforementioned BC subtype.

High-Aspect-Ratio Nanostructured Surfaces as Biological Metamaterials

Materials patterned with high-aspect-ratio nanostructures have features on similar length scales to cellular components. These surfaces are an extreme topography on the cellular level and have become useful tools for perturbing and sensing the cellular environment. Motivation comes from the ability of high-aspect-ratio nanostructures to deliver cargoes into cells and tissues, access the intracellular environment, and control cell behavior. These structures directly perturb cells' ability to sense and respond to external forces, influencing cell fate, and enabling new mechanistic studies. Through careful design of their nanoscale structure, these systems act as biological metamaterials, eliciting unusual biological responses. While predominantly used to interface eukaryotic cells, there is growing interest in nonanimal and prokaryotic cell interfacing. Both experimental and theoretical studies have attempted to develop a mechanistic understanding for the observed behaviors, predominantly focusing on the cell-nanostructure interface. This review considers how high-aspect-ratio nanostructured surfaces are used to both stimulate and sense biological systems.

Engineering Biomaterials with Micro/Nanotechnologies for Cell Reprogramming

Cell reprogramming is a revolutionized biotechnology that offers a powerful tool to engineer cell fate and function for regenerative medicine, disease modeling, drug discovery, and beyond. Leveraging advances in biomaterials and micro/nanotechnologies can enhance the reprogramming performance in vitro and in vivo through the development of delivery strategies and the control of biophysical and biochemical cues. In this review, we present an overview of the state-of-the-art technologies for cell reprogramming and highlight the recent breakthroughs in engineering biomaterials with micro/nanotechnologies to improve reprogramming efficiency and quality. Finally, we discuss future directions and challenges for reprogramming technologies and clinical translation.

Mechanisms of Arterial Stiffening: From Mechanotransduction to Epigenetics

Arterial stiffness is a major independent risk factor for cardiovascular complications causing isolated systolic hypertension and increased pulse pressure in the microvasculature of target organs. Stiffening of the arterial wall is determined by common mechanisms including reduced elastin/collagen ratio, production of elastin cross-linking, reactive oxygen species-induced inflammation, calcification, vascular smooth muscle cell stiffness, and endothelial dysfunction. This brief review will discuss current biological mechanisms by which other cardiovascular risk factors (eg, aging, hypertension, diabetes mellitus, and chronic kidney disease) cause arterial stiffness, with a particular focus on recent advances regarding nuclear mechanotransduction, mitochondrial oxidative stress, metabolism and dyslipidemia, genome mutations, and epigenetics. Targeting these different molecular pathways at different time of cardiovascular risk factor exposure may be a novel approach for discovering drugs to reduce arterial stiffening without affecting artery strength and normal remodeling.

Tendon and ligament mechanical loading in the pathogenesis of inflammatory arthritis

Mechanical loading is an important factor in musculoskeletal health and disease. Tendons and ligaments require physiological levels of mechanical loading to develop and maintain their tissue architecture, a process that is achieved at the cellular level through mechanotransduction-mediated fine tuning of the extracellular matrix by tendon and ligament stromal cells. Pathological levels of force represent a biological (mechanical) stress that elicits an immune system-mediated tissue repair pathway in tendons and ligaments. The biomechanics and mechanobiology of tendons and ligaments form the basis for understanding how such tissues sense and respond to mechanical force, and the anatomical extent of several mechanical stress-related disorders in tendons and ligaments overlaps with that of chronic inflammatory arthritis in joints. The role of mechanical stress in 'overuse' injuries, such as tendinopathy, has long been known, but mechanical stress is now also emerging as a possible trigger for some forms of chronic inflammatory arthritis, including spondyloarthritis and rheumatoid arthritis. Thus, seemingly diverse diseases of the musculoskeletal system might have similar mechanisms of immunopathogenesis owing to conserved responses to mechanical stress.

DLC1 is a direct target of activated YAP/TAZ that drives collective migration and sprouting angiogenesis

Endothelial YAP/TAZ (YAP is also known as YAP1, and TAZ as WWTR1) signaling is crucial for sprouting angiogenesis and vascular homeostasis. However, the underlying molecular mechanisms that explain how YAP/TAZ control the vasculature remain unclear. This study reveals that the focal adhesion protein deleted-in-liver-cancer 1 (DLC1) is a direct transcriptional target of the activated YAP/TAZ-TEAD complex. We find that substrate stiffening and VEGF stimuli promote expression of DLC1 in endothelial cells. In turn, DLC1 expression levels are YAP and TAZ dependent, and constitutive activation of YAP is sufficient to drive DLC1 expression. DLC1 is needed to limit F-actin fiber formation, integrin-based focal adhesion lifetime and integrin-mediated traction forces. Depletion of endothelial DLC1 strongly perturbs cell polarization in directed collective migration and inhibits the formation of angiogenic sprouts. Importantly, ectopic expression of DLC1 is sufficient to restore migration and angiogenic sprouting in YAP-depleted cells. Together, these findings point towards a crucial and prominent role for DLC1 in YAP/TAZ-driven endothelial adhesion remodeling and collective migration during angiogenesis.This article has an associated First Person interview with the first author of the paper.

Nuclear inclusions of pathogenic ataxin-1 induce oxidative stress and perturb the protein synthesis machinery

Spinocerebellar ataxia type-1 (SCA1) is caused by an abnormally expanded polyglutamine (polyQ) tract in ataxin-1. These expansions are responsible for protein misfolding and self-assembly into intranuclear inclusion bodies (IIBs) that are somehow linked to neuronal death. However, owing to lack of a suitable cellular model, the downstream consequences of IIB formation are yet to be resolved. Here, we describe a nuclear protein aggregation model of pathogenic human ataxin-1 and characterize IIB effects. Using an inducible Sleeping Beauty transposon system, we overexpressed the ATXN1(Q82) gene in human mesenchymal stem cells that are resistant to the early cytotoxic effects caused by the expression of the mutant protein. We characterized the structure and the protein composition of insoluble polyQ IIBs which gradually occupy the nuclei and are responsible for the generation of reactive oxygen species. In response to their formation, our transcriptome analysis reveals a cerebellum-specific perturbed protein interaction network, primarily affecting protein synthesis. We propose that insoluble polyQ IIBs cause oxidative and nucleolar stress and affect the assembly of the ribosome by capturing or down-regulating essential components. The inducible cell system can be utilized to decipher the cellular consequences of polyQ protein aggregation. Our strategy provides a broadly applicable methodology for studying polyQ diseases.

Hemidesmosomes modulate force generation via focal adhesions

Hemidesmosomes are specialized cell-matrix adhesion structures that are associated with the keratin cytoskeleton. Although the adhesion function of hemidesmosomes has been extensively studied, their role in mechanosignaling and transduction remains largely unexplored. Here, we show that keratinocytes lacking hemidesmosomal integrin α6β4 exhibit increased focal adhesion formation, cell spreading, and traction-force generation. Moreover, disruption of the interaction between α6β4 and intermediate filaments or laminin-332 results in similar phenotypical changes. We further demonstrate that integrin α6β4 regulates the activity of the mechanosensitive transcriptional regulator YAP through inhibition of Rho-ROCK-MLC- and FAK-PI3K-dependent signaling pathways. Additionally, increased tension caused by impaired hemidesmosome assembly leads to a redistribution of integrin αVβ5 from clathrin lattices to focal adhesions. Our results reveal a novel role for hemidesmosomes as regulators of cellular mechanical forces and establish the existence of a mechanical coupling between adhesion complexes.

The HIPPO pathway in gynecological malignancies

The Hippo pathway has been initially discovered by screening genes that regulate organ size in Drosophila. Recent studies have highlighted the role of the Hippo pathway in controlling organ size, tissue homeostasis and regeneration, and signaling dysregulation, especially the overactivation of the transcriptional coactivator YAP/TAZ, which leads to uncontrolled cell growth and malignant transformation. The core components of the Hippo pathway may initiate tumorigenesis by inducing tumor stem cells and proliferation, ultimately leading to metastasis and drug resistance, which occurs extensively in gynecological malignancies, including cervical cancer, ovarian cancer, and endometrial cancer. In this review, we attempt to systematically summarize recent progress in our understanding of the mechanism of Hippo pathway regulation in tumorigenesis and the mechanisms that underlie alterations during gynecological malignancies, as well as new therapeutic strategies.

Three-dimensional imaging of cell and extracellular matrix elasticity using quantitative micro-elastography

Recent studies in mechanobiology have revealed the importance of cellular and extracellular mechanical properties in regulating cellular function in normal and disease states. Although it is established that cells should be investigated in a three-dimensional (3-D) environment, most techniques available to study mechanical properties on the microscopic scale are unable to do so. In this study, for the first time, we present volumetric images of cellular and extracellular elasticity in 3-D biomaterials using quantitative micro-elastography (QME). We achieve this by developing a novel strain estimation algorithm based on 3-D linear regression to improve QME system resolution. We show that QME can reveal elevated elasticity surrounding human adipose-derived stem cells (ASCs) embedded in soft hydrogels. We observe, for the first time in 3-D, further elevation of extracellular elasticity around ASCs with overexpressed TAZ; a mechanosensitive transcription factor which regulates cell volume. Our results demonstrate that QME has the potential to study the effects of extracellular mechanical properties on cellular functions in a 3-D micro-environment.

Direct evidence that tumor cells soften when navigating confined spaces

The mechanical properties of cells strongly regulate many physiological and pathological processes. For example, in cancer, invasive and metastatic tumor cells have often been reported to be softer than nontumor cells, raising speculation that cancer cells might adaptively soften to facilitate migration through narrow tissue spaces. Despite growing interest in targeting cell softening to impede invasion and metastasis, it remains to be directly demonstrated that tumor cells soften as they migrate through confined spaces. Here, we address this open question by combining topographically patterned substrates with atomic force microscopy (AFM). Using a polydimethylsiloxane open-roof microdevice featuring tapered, fibronectin-coated channels, we followed the migration of U2OS cells through various stages of confinement while simultaneously performing AFM indentation. As cells progress from unconfined migration to fully confined migration, cells soften and exclude Yes-associated protein from the nucleus. Superresolution imaging reveals that confinement induces remodeling of actomyosin stress fiber architecture. Companion studies with flat one-dimensional microlines indicate that the changes in cytoarchitecture and mechanics are intrinsically driven by topographical confinement rather than changes in cellular aspect ratio. Our studies represent among the most direct evidence to date that tumor cells soften during confined migration and support cell softening as a mechanoadaptive mechanism during invasion.

YAP/TAZ functions and their regulation at a glance

YAP and TAZ proteins are transcriptional coactivators encoded by paralogous genes, which shuttle between the cytoplasm and the nucleus in response to multiple inputs, including the Hippo pathway. In the nucleus, they pair with DNA-binding factors of the TEAD family to regulate gene expression. Nuclear YAP/TAZ promote cell proliferation, organ overgrowth, survival to stress and dedifferentiation of post-mitotic cells into their respective tissue progenitors. YAP/TAZ are required for growth of embryonic tissues, wound healing and organ regeneration, where they are activated by cell-intrinsic and extrinsic cues. Surprisingly, this activity is dispensable in many adult self-renewing tissues, where YAP/TAZ are constantly kept in check. YAP/TAZ lay at the center of a complex regulatory network including cell-autonomous factors but also cell- and tissue-level structural features such as the mechanical properties of the cell microenvironment, the establishment of cell-cell junctions and of basolateral tissue polarity. Enhanced levels and activity of YAP/TAZ are observed in many cancers, where they sustain tumor growth, drug resistance and malignancy. In this Cell Science at a Glance article and the accompanying poster, we review the biological functions of YAP/TAZ and their regulatory mechanisms, and highlight their position at the center of a complex signaling network.

A neuroglia-based interpretation of glaucomatous neuroretinal rim thinning in the optic nerve head

Neuroretinal rim thinning (NRR) is a characteristic glaucomatous optic disc change. However, the precise mechanism of the rim thinning has not been completely elucidated. This review focuses on the structural role of the glioarchitecture in the formation of the glaucomatous NRR thinning. The NRR is a glia-framed structure, with honeycomb geometry and mechanically reinforced astrocyte processes along the transverse plane. When neural damage selectively involves the neuron and spares the glia, the gross structure of the tissue is preserved. The disorganization and loss of the glioarchitecture are the two hallmarks of optic nerve head (ONH) remodeling in glaucoma that leads to the thinning of NRR tissue upon axonal loss. This is in contrast to most non-glaucomatous optic neuropathies with optic disc pallor where hypertrophy of the glioarchitecture is associated with the seemingly absent optic disc cupping. Arteritic anterior ischemic optic neuropathy is an exception where pan-necrosis of ONH tissue leads to NRR thinning. Milder ischemia indicates selective neuronal loss that spares glia in non-arteritic anterior ischemic optic neuropathy. The biological reason is the heterogeneous glial response determined by the site, type, and severity of the injury. The neuroglial interpretation explains how the cellular changes underlie the clinical findings. Updated understandings on glial responses illustrate the mechanical, microenvironmental, and microglial modulation of activated astrocytes in glaucoma. Findings relevant to the possible mechanism of the astrocyte death in advanced glaucoma are also emerging. Ultimately, a better understanding of glaucomatous glial response may lead to glia-targeting neuroprotection in the future.

Dysfunctional Mechanotransduction through the YAP/TAZ/Hippo Pathway as a Feature of Chronic Disease

In order to ascertain their external environment, cells and tissues have the capability to sense and process a variety of stresses, including stretching and compression forces. These mechanical forces, as experienced by cells and tissues, are then converted into biochemical signals within the cell, leading to a number of cellular mechanisms being activated, including proliferation, differentiation and migration. If the conversion of mechanical cues into biochemical signals is perturbed in any way, then this can be potentially implicated in chronic disease development and processes such as neurological disorders, cancer and obesity. This review will focus on how the interplay between mechanotransduction, cellular structure, metabolism and signalling cascades led by the Hippo-YAP/TAZ axis can lead to a number of chronic diseases and suggest how we can target various pathways in order to design therapeutic targets for these debilitating diseases and conditions.

Modular mimicry and engagement of the Hippo pathway by Marburg virus VP40: Implications for filovirus biology and budding

Ebola (EBOV) and Marburg (MARV) are members of the Filoviridae family, which continue to emerge and cause sporadic outbreaks of hemorrhagic fever with high mortality rates. Filoviruses utilize their VP40 matrix protein to drive virion assembly and budding, in part, by recruitment of specific WW-domain-bearing host proteins via its conserved PPxY Late (L) domain motif. Here, we screened an array of 115 mammalian, bacterially expressed and purified WW-domains using a PPxY-containing peptide from MARV VP40 (mVP40) to identify novel host interactors. Using this unbiased approach, we identified Yes Associated Protein (YAP) and Transcriptional co-Activator with PDZ-binding motif (TAZ) as novel mVP40 PPxY interactors. YAP and TAZ function as downstream transcriptional effectors of the Hippo signaling pathway that regulates cell proliferation, migration and apoptosis. We demonstrate that ectopic expression of YAP or TAZ along with mVP40 leads to significant inhibition of budding of mVP40 VLPs in a WW-domain/PPxY dependent manner. Moreover, YAP colocalized with mVP40 in the cytoplasm, and inhibition of mVP40 VLP budding was more pronounced when YAP was localized predominantly in the cytoplasm rather than in the nucleus. A key regulator of YAP nuclear/cytoplasmic localization and function is angiomotin (Amot); a multi-PPxY containing protein that strongly interacts with YAP WW-domains. Interestingly, we found that expression of PPxY-containing Amot rescued mVP40 VLP egress from either YAP- or TAZ-mediated inhibition in a PPxY-dependent manner. Importantly, using a stable Amot-knockdown cell line, we found that expression of Amot was critical for efficient egress of mVP40 VLPs as well as egress and spread of authentic MARV in infected cell cultures. In sum, we identified novel negative (YAP/TAZ) and positive (Amot) regulators of MARV VP40-mediated egress, that likely function in part, via competition between host and viral PPxY motifs binding to modular host WW-domains. These findings not only impact our mechanistic understanding of virus budding and spread, but also may impact the development of new antiviral strategies.

By screening an array of 115 mammalian WW-domains with the PPxY motif from MARV VP40 (mVP40), we identified YAP1 and TAZ, transcriptional effectors of the Hippo pathway, as mVP40 interactors, and demonstrated that ectopically expressed YAP1 or TAZ inhibited budding of mVP40 virus-like particles (VLPs) in a WW-domain/PPxY dependent manner. Angiomotin (Amot), a multi-PPxY containing regulator of YAP1 nuclear/cytoplasmic localization and function, rescued mVP40 VLP egress from either YAP1- or TAZ-mediated inhibition in a PPxY-dependent manner. Indeed, endogenous Amot expression was critical for egress of mVP40 VLPs and authentic MARV. In sum, we have revealed a link between the Hippo pathway and filovirus egress by identifying negative (YAP/TAZ) and positive (Amot) regulators of MARV VP40-mediated egress.

Evasion of apoptosis by myofibroblasts: a hallmark of fibrotic diseases

Organ fibrosis is a lethal outcome of autoimmune rheumatic diseases such as systemic sclerosis. Myofibroblasts are scar-forming cells that are ultimately responsible for the excessive synthesis, deposition and remodelling of extracellular matrix proteins in fibrosis. Advances have been made in our understanding of the mechanisms that keep myofibroblasts in an activated state and control myofibroblast functions. However, the mechanisms that help myofibroblasts to persist in fibrotic tissues remain poorly understood. Myofibroblasts evade apoptosis by activating molecular mechanisms in response to pro-survival biomechanical and growth factor signals from the fibrotic microenvironment, which can ultimately lead to the acquisition of a senescent phenotype. Growing evidence suggests that myofibroblasts and senescent myofibroblasts, rather than being resistant to apoptosis, are actually primed for apoptosis owing to concomitant activation of cell death signalling pathways; these cells are poised to apoptose when survival pathways are inhibited. This knowledge of apoptotic priming has paved the way for new therapies that trigger apoptosis in myofibroblasts by blocking pro-survival mechanisms, target senescent myofibroblast for apoptosis or promote the reprogramming of myofibroblasts into scar-resolving cells. These novel strategies are not only poised to prevent progressive tissue scarring, but also have the potential to reverse established fibrosis and to regenerate chronically injured tissues.

The spectraplakin Dystonin antagonizes YAP activity and suppresses tumourigenesis

Aberrant expression of the Spectraplakin Dystonin (DST) has been observed in various cancers, including those of the breast. However, little is known about its role in carcinogenesis. In this report, we demonstrate that Dystonin is a candidate tumour suppressor in breast cancer and provide an underlying molecular mechanism. We show that in MCF10A cells, Dystonin is necessary to restrain cell growth, anchorage-independent growth, self-renewal properties and resistance to doxorubicin. Strikingly, while Dystonin maintains focal adhesion integrity, promotes cell spreading and cell-substratum adhesion, it prevents Zyxin accumulation, stabilizes LATS and restricts YAP activation. Moreover, treating DST-depleted MCF10A cells with the YAP inhibitor Verteporfin prevents their growth. In vivo, the Drosophila Dystonin Short stop also restricts tissue growth by limiting Yorkie activity. As the two Dystonin isoforms BPAG1eA and BPAG1e are necessary to inhibit the acquisition of transformed features and are both downregulated in breast tumour samples and in MCF10A cells with conditional induction of the Src proto-oncogene, they could function as the predominant Dystonin tumour suppressor variants in breast epithelial cells. Thus, their loss could deem as promising prognostic biomarkers for breast cancer.

Volume Adaptation Controls Stem Cell Mechanotransduction

Recent studies have found discordant mechanosensitive outcomes when comparing 2D and 3D, highlighting the need for tools to study mechanotransduction in 3D across a wide spectrum of stiffness. A gelatin methacryloyl (GelMA) hydrogel with a continuous stiffness gradient ranging from 5 to 38 kPa was developed to recapitulate physiological stiffness conditions. Adipose-derived stem cells (ASCs) were encapsulated in this hydrogel, and their morphological characteristics and expression of both mechanosensitive proteins (Lamin A, YAP, and MRTFa) and differentiation markers (PPARγ and RUNX2) were analyzed. Low-stiffness regions (∼8 kPa) permitted increased cellular and nuclear volume and enhanced mechanosensitive protein localization in the nucleus. This trend was reversed in high stiffness regions (∼30 kPa), where decreased cellular and nuclear volumes and reduced mechanosensitive protein nuclear localization were observed. Interestingly, cells in soft regions exhibited enhanced osteogenic RUNX2 expression, while those in stiff regions upregulated the adipogenic regulator PPARγ, suggesting that volume, not substrate stiffness, is sufficient to drive 3D stem cell differentiation. Inhibition of myosin II (Blebbistatin) and ROCK (Y-27632), both key drivers of actomyosin contractility, resulted in reduced cell volume, especially in low-stiffness regions, causing a decorrelation between volume expansion and mechanosensitive protein localization. Constitutively active and inactive forms of the canonical downstream mechanotransduction effector TAZ were stably transfected into ASCs. Activated TAZ resulted in higher cellular volume despite increasing stiffness and a consistent, stiffness-independent translocation of YAP and MRTFa into the nucleus. Thus, volume adaptation as a function of 3D matrix stiffness can control stem cell mechanotransduction and differentiation.

Stem Cell Mechanosensation on Gelatin Methacryloyl (GelMA) Stiffness Gradient Hydrogels

Stiffness gradient hydrogels are a useful platform for studying mechanical interactions between cells and their surrounding environments. Here, we developed linear stiffness gradient hydrogels by controlling the polymerization of gelatin methacryloyl (GelMA) via differential UV penetration with a gradient photomask. Based on previous observations, a stiffness gradient GelMA hydrogel was created ranging from ~ 4 to 13 kPa over 15 mm (0.68 kPa/mm), covering the range of physiological tissue stiffness from fat to muscle, thereby allowing us to study stem cell mechanosensation and differentiation. Adipose-derived stem cells on these gradient hydrogels showed no durotaxis, which allowed for the screening of mechanomarker expression without confounding directed migration effects. In terms of morphological markers, the cell aspect ratio showed a clear positive correlation to the underlying substrate stiffness, while no significant correlation was found in cell size, nuclear size, or nuclear aspect ratio. Conversely, expression of mechanomarkers (i.e., Lamin A, YAP, and MRTFa) all showed a highly significant correlation to stiffness, which could be disrupted via inhibition of non-muscle myosin or Rho/ROCK signalling. Furthermore, we showed that cells plated on stiffer regions became stiffer themselves, and that stem cells showed stiffness-dependent differentiation to fat or muscle as has been previously reported in the literature.

The Hippo Pathway, YAP/TAZ, and the Plasma Membrane

The plasma membrane allows the cell to sense and adapt to changes in the extracellular environment by relaying external inputs via intracellular signaling networks. One central cellular signaling pathway is the Hippo pathway, which regulates homeostasis and plays chief roles in carcinogenesis and regenerative processes. Recent studies have found that mechanical stimuli and diffusible chemical components can regulate the Hippo pathway primarily through receptors embedded in the plasma membrane. Morphologically defined structures within the plasma membrane, such as cellular junctions, focal adhesions, primary cilia, caveolae, clathrin-coated pits, and plaques play additional key roles. Here, we discuss recent evidence highlighting the importance of these specialized plasma membrane domains in cellular feedback via the Hippo pathway.

Mechanosensitive transcriptional coactivators MRTF-A and YAP/TAZ regulate nucleus pulposus cell phenotype through cell shape

Cells of the adult nucleus pulposus (NP) are critically important in maintaining overall disc health and function. NP cells reside in a soft, gelatinous matrix that dehydrates and becomes increasingly fibrotic with age. Such changes result in physical cues of matrix stiffness that may be potent regulators of NP cell phenotype and may contribute to a transition toward a senescent and fibroblastic NP cell with a limited capacity for repair. Here, we investigate the mechanosignaling cues generated from changes in matrix stiffness in directing NP cell phenotype and identify mechanisms that can potentially preserve a biosynthetically active, juvenile NP cell phenotype. Using a laminin-functionalized polyethylene glycol hydrogel, we show that when NP cells form rounded, multicell clusters, they are able to maintain cytosolic localization of myocardin-related transcription factor (MRTF)-A, a coactivator of serum-response factor (SRF), known to promote fibroblast-like behaviors in many cells. Upon preservation of a rounded shape, human NP cells similarly showed cytosolic retention of transcriptional coactivator Yes-associated protein (YAP) and its paralogue PDZ-binding motif (TAZ) with associated decline in activation of its transcription factor TEA domain family member-binding domain (TEAD). When changes in cell shape occur, leading to a more spread, fibrotic morphology associated with stronger F-actin alignment, SRF and TEAD are up-regulated. However, targeted deletion of either cofactor was not sufficient to overcome shape-mediated changes observed in transcriptional activation of SRF or TEAD. Findings show that substrate stiffness-induced promotion of F-actin alignment occurs concomitantly with a flattened, spread morphology, decreased NP marker expression, and reduced biosynthetic activity. This work indicates cell shape is a stronger indicator of SRF and TEAD mechanosignaling pathways than coactivators MRTF-A and YAP/TAZ, respectively, and may play a role in the degeneration-associated loss of NP cellularity and phenotype.-Fearing, B. V., Jing, L., Barcellona, M. N., Witte, S. E., Buchowski, J. M., Zebala, L. P., Kelly, M. P., Luhmann, S., Gupta, M. C., Pathak, A., Setton, L. A. Mechanosensitive transcriptional coactivators MRTF-A and YAP/TAZ regulate nucleus pulposus cell phenotype through cell shape.

Sub-Micropillar Spacing Modulates the Spatial Arrangement of Mouse MC3T3-E1 Osteoblastic Cells

Surface topography is one of the main factors controlling cell responses on implanted devices and a proper definition of the characteristics that optimize cell behavior may be crucial to improve the clinical performances of these implants. Substrate geometry is known to affect cell shape, as cells try to optimize their adhesion by adapting to the irregularities beneath, and this in turn profoundly affects their activity. In the present study, we cultured murine calvaria MC3T3-E1 cells on surfaces with pillars arranged as hexagons with two different spacings and observed their morphology during adhesion and growth. Cells on these highly ordered substrates attached and proliferated effectively, showing a marked preference for minimizing the inter-pillar distance, by following specific pathways across adjacent pillars and displaying consistent morphological modules. Moreover, cell behavior appeared to follow tightly controlled patterns of extracellular protein secretion, which preceded and matched cells and, on a sub-cellular level, cytoplasmic orientation. Taken together, these results outline the close integration of surface features, extracellular proteins alignment and cell arrangement, and provide clues on how to control and direct cell spatial order and cell morphology by simply acting on inter-pillar spacing.

Nanoparticle-Based Hybrid Scaffolds for Deciphering the Role of Multimodal Cues in Cardiac Tissue Engineering

Myocardial microenvironment plays a decisive role in guiding the function and fate of cardiomyocytes, and engineering this extracellular niche holds great promise for cardiac tissue regeneration. Platforms utilizing hybrid hydrogels containing various types of conductive nanoparticles have been a critical tool for constructing engineered cardiac tissues with outstanding mechanical integrity and improved electrophysiological properties. However, there has been no attempt to directly compare the efficacy of these hybrid hydrogels and decipher the mechanisms behind how these platforms differentially regulate cardiomyocyte behavior. Here, we employed gelatin methacryloyl (GelMA) hydrogels containing three different types of carbon-based nanoparticles: carbon nanotubes (CNTs), graphene oxide (GO), and reduced GO (rGO), to investigate the influence of these hybrid scaffolds on the structural organization and functionality of cardiomyocytes. Using immunofluorescent staining for assessing cellular organization and proliferation, we showed that electrically conductive scaffolds (CNT- and rGO-GelMA compared to relatively nonconductive GO-GelMA) played a significant role in promoting desirable morphology of cardiomyocytes and elevated the expression of functional cardiac markers, while maintaining their viability. Electrophysiological analysis revealed that these engineered cardiac tissues showed distinct cardiomyocyte phenotypes and different levels of maturity based on the substrate (CNT-GelMA: ventricular-like, GO-GelMA: atrial-like, and rGO-GelMA: ventricular/atrial mixed phenotypes). Through analysis of gene-expression patterns, we uncovered that the engineered cardiac tissues matured on CNT-GelMA and native cardiac tissues showed comparable expression levels of maturation markers. Furthermore, we demonstrated that engineered cardiac tissues matured on CNT-GelMA have increased functionality through integrin-mediated mechanotransduction (via YAP/TAZ) in contrast to cardiomyocytes cultured on rGO-GelMA.

Lineage Commitment, Signaling Pathways, and the Cytoskeleton Systems in Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) from adult tissues are promising candidates for personalized cell therapy and tissue engineering. Significant progress was achieved in our understanding of the regulation of MSCs proliferation and differentiation by different cues during the past years. Proliferation and differentiation of MSCs are sensitive to the extracellular matrix (ECM) properties, physical cues, and chemical signaling. Sheath stress, matrix stiffness, surface adhesiveness, and micro- and nanotopography define cell shape and dictate lineage commitment of MSCs even in the absence of specific chemical signals. We discuss mechanotransduction as the major route from ECM through the cytoskeleton toward signaling pathways and gene expression. All components of the cytoskeleton from primary cilium and focal adhesions (FAs) to actin, microtubules (MTs), and intermediate filaments (IFs) are involved in the mechanotransduction. Differentiation of MSCs is regulated via the complex network of interrelated signaling pathways, including RhoA/ROCK, Akt/Erk, and YAP/TAZ effectors of Hippo pathway. These pathways could be regulated both by chemical and mechanical stimuli. Attenuation of these pathways in MSCs results in specific changes in FAs and actin cytoskeleton. Besides, differentiation of MSCs affects MTs and IFs. Recent findings highlight the role of intranuclear actin in the regulation of transcription factors in response to mechanical environmental stimuli. Alterations of cytoskeletal components reflect the MSC senescence state and their migratory capacity. In this review, we discuss the relationships between the molecular interactions in signaling pathways and morphological response of cytoskeletal components and reveal the complex interrelations between cytoskeleton systems and signaling pathways during lineage commitment of MSCs. Impact Statement This review describes the complex network of relationships between mechanical and biochemical stimuli in mesenchymal stem cells (MSC) and their balance which defines the morphological changes of cell shape due to rearrangement of cytoskeletal systems during lineage commitment of MSCs.

Cell Adhesion by Integrins

Integrins are heterodimeric cell surface receptors ensuring the mechanical connection between cells and the extracellular matrix. In addition to the anchorage of cells to the extracellular matrix, these receptors have critical functions in intracellular signaling, but are also taking center stage in many physiological and pathological conditions. In this review, we provide some historical, structural, and physiological notes so that the diverse functions of these receptors can be appreciated and put into the context of the emerging field of mechanobiology. We propose that the exciting journey of the exploration of these receptors will continue for at least another new generation of researchers.

Five Piconewtons: The Difference between Osteogenic and Adipogenic Fate Choice in Human Mesenchymal Stem Cells

The ability of mesenchymal stem cells to sense nanoscale variations in extracellular matrix (ECM) compositions in their local microenvironment is crucial to their survival and their fate; however, the underlying molecular mechanisms defining how such fates are temporally modulated remain poorly understood. In this work, we have utilized self-assembled block copolymer surfaces to present nanodomains of an adhesive peptide found in many ECM proteins at different lateral spacings (from 30 to 60 nm) and studied the temporal response (2 h to 14 days) of human mesenchymal stem cells (hMSCs) using a panel of real-time localization and activity biosensors. Our findings revealed that within the first 4 to 24 h postadhesion and spreading, hMSCs on smaller nanodomain spacings recruit more activated FAK and Src proteins to produce larger, longer-lived, and increased numbers of focal adhesions (FAs). The adhesions formed on smaller nanospacings rapidly recruit higher amounts of nonmuscle myosin IIA and vinculin and experience tension forces (by >5 pN/FA) significantly higher than those observed on larger nanodomain spacings. The transmission of higher levels of tension into the cytoskeleton at short times was accompanied by higher Rac1, cytosolic β-catenin, and nuclear localization of YAP/TAZ and RUNX2, which together biased the commitment of hMSCs to an osteogenic fate. This investigation provides mechanistic insights to confirm that smaller lateral spacings of adhesive nanodomains alter hMSC mechanosensing and biases mechanotransduction at short times via differential coupling of FAK/Src/Rac1/myosin IIA/YAP/TAZ signaling pathways to support longer-term changes in stem cell differentiation and state.

In vivo engineered extracellular matrix scaffolds with instructive niches for oriented tissue regeneration

Implanted scaffolds with inductive niches can facilitate the recruitment and differentiation of host cells, thereby enhancing endogenous tissue regeneration. Extracellular matrix (ECM) scaffolds derived from cultured cells or natural tissues exhibit superior biocompatibility and trigger favourable immune responses. However, the lack of hierarchical porous structure fails to provide cells with guidance cues for directional migration and spatial organization, and consequently limit the morpho-functional integration for oriented tissues. Here, we engineer ECM scaffolds with parallel microchannels (ECM-C) by subcutaneous implantation of sacrificial templates, followed by template removal and decellularization. The advantages of such ECM-C scaffolds are evidenced by close regulation of in vitro cell activities, and enhanced cell infiltration and vascularization upon in vivo implantation. We demonstrate the versatility and flexibility of these scaffolds by regenerating vascularized and innervated neo-muscle, vascularized neo-nerve and pulsatile neo-artery with functional integration. This strategy has potential to yield inducible biomaterials with applications across tissue engineering and regenerative medicine.

Extracellular matrix (ECM) is an ideal scaffold for tissue engineering but tends to lack hierarchical structure. Here the authors implant sacrificial templates subcutaneously to build an organised ECM scaffold, and following template removal and decellularisation use these scaffolds to create functionally integrated muscle, nerve and artery in vivo.

Elevated BMP and Mechanical Signaling Through YAP1/RhoA Poises FOP Mesenchymal Progenitors for Osteogenesis

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease characterized by the formation of extraskeletal bone, or heterotopic ossification (HO), in soft connective tissues such as skeletal muscle. All familial and sporadic cases with a classic clinical presentation of FOP carry a gain-of-function mutation (R206H; c.617 G > A) in ACVR1, a cell surface receptor that mediates bone morphogenetic protein (BMP) signaling. The BMP signaling pathway is recognized for its chondro/osteogenic-induction potential, and HO in FOP patients forms ectopic but qualitatively normal endochondral bone tissue through misdirected cell fate decisions by tissue-resident mesenchymal stem cells. In addition to biochemical ligand-receptor signaling, mechanical cues from the physical environment are transduced to activate intracellular signaling, a process known as mechanotransduction, and can influence cell fates. Utilizing an established mesenchymal stem cell model of mouse embryonic fibroblasts (MEFs) from the Acvr1R206H/+ mouse model that mimics the human disease, we demonstrated that activation of the mechanotransductive effectors Rho/ROCK and YAP1 are increased in Acvr1R206H/+ cells. We show that on softer substrates, a condition associated with low mechanical signaling, the morphology of Acvr1R206H/+ cells is similar to the morphology of control Acvr1+/+ cells on stiffer substrates, a condition that activates mechanotransduction. We further determined that Acvr1R206H/+ cells are poised for osteogenic differentiation, expressing increased levels of chondro/osteogenic markers compared with Acvr1+/+ cells. We also identified increased YAP1 nuclear localization in Acvr1R206H/+ cells, which can be rescued by either BMP inhibition or Rho antagonism. Our results establish RhoA and YAP1 signaling as modulators of mechanotransduction in FOP and suggest that aberrant mechanical signals, combined with and as a result of the increased BMP pathway signaling through mutant ACVR1, lead to misinterpretation of the cellular microenvironment and a heightened sensitivity to mechanical stimuli that promotes commitment of Acvr1R206H/+ progenitor cells to chondro/osteogenic lineages.

Varying solvent type modulates collagen coating and stem cell mechanotransduction on hydrogel substrates

Type I collagen is the most abundant extracellular matrix protein in the human body and is commonly used as a biochemical ligand for hydrogel substrates to support cell adhesion in mechanotransduction studies. Previous protocols for conjugating collagen I have used different solvents; yet, how varying solvent pH and composition impacts the efficiency and distribution of these collagen I coatings remains unknown. Here, we examine the effect of varying solvent pH and type on the efficiency and distribution of collagen I coatings on polyacrylamide hydrogels. We further evaluate the effects of varying solvent on mechanotransduction of human mesenchymal stem cells (MSCs) by characterizing cell spreading and localization of Yes-Associated Protein (YAP), a key transcriptional regulator of mechanotransduction. Increasing solvent pH to 5.2 and above increased the heterogeneity of coating with collagen bundle formation. Collagen I coating highly depends on the solvent type, with acetic acid leading to the highest conjugation efficiency and most homogeneous coating. Compared to HEPES or phosphate-buffered saline buffer, acetic acid-dissolved collagen I coatings substantially enhance MSC adhesion and spreading on both glass and polyacrylamide hydrogel substrates. When acetic acid was used for collagen coatings, even the low collagen concentration (1 μg/ml) induced robust MSC spreading and nuclear YAP localization on both soft (3 kPa) and stiff (38 kPa) substrates. Depending on the solvent type, stiffness-dependent nuclear YAP translocation occurs at a different collagen concentration. Together, the results from this study validate the solvent type as an important parameter to consider when using collagen I as the biochemical ligand to support cell adhesion.